1. Field of the Invention
The present invention relates to integrated circuits incorporating shielded circuit structures, and more particularly to such integrated circuits incorporating inductor structures.
2. Description of the Related Art
Many modern integrated circuit devices require extremely stable oscillators to achieve desired specifications. Crystal oscillators may be employed, but typically require an off-chip crystal mounted elsewhere on a printed-wiring-board. LC oscillators offer the potential advantage of being able to incorporate such an oscillator on-chip.
To achieve an oscillator having extremely low-jitter generation, a high-Q LC oscillator is typically required. For example, a Q>30 may be required for certain applications. It is difficult to achieve such a high Q with conventional on-chip spiral inductors using the conductor and dielectric layer compositions and thicknesses which are typically encountered in traditional integrated circuit processes. For example, such on-chip spiral inductors typically achieve a Q no higher than 10–15, which is limited largely by substrate loss due to induced eddy currents in the semiconductor substrate. In addition, such spiral inductors are susceptible to electromagnetic interference from external sources of noise.
Additional improvements in high-Q LC oscillators are desired to achieve extremely stable oscillators, particularly for use as low-jitter clock sources.